Liam Treadwell will miss the ride on Tenor Nivernais in Saturday's Randox Health Grand National after suffering a heavy fall at Aintree on Friday.
Treadwell, who won the 2009 Grand National aboard 100-1 shot Mon Mome, only made it as far as the first fence before falling from Bright New Dawn in the Topham Chase.
The field had to bypass the obstacle second time around as Treadwell was being attended to by medics.
Venetia Williams, trainer of both Bright New Dawn and Tenor Nivernais, was initially hopeful Treadwell would be fit for the Aintree spectacular, but later discovered he would not be available.
Leading jockey Aidan Coleman will take over in the saddle aboard the 10-year-old.
Williams said: "Liam was taken to hospital for precautionary checks and has let me know that unfortunately, he is out for Saturday.
"They are keeping in overnight and he says there is no way he's going to be able to ride.
"The plan is for Aidan Coleman to ride Tenor Nivernais."
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